High pressure sodium discharge lamp

ABSTRACT

A high pressure sodium vapour lamp having a fill consisting of sodium, mercury and xenon in an arc tube with a sodium weight portion within the sodium-mercury-amalgam of approx. 12% to approx. 20%, having a xenon filling pressure in the cold state between approx. 180 Torr and approx. 350 Torr, having a D-line reversal width (distance of the tops of both wings of the sodium-D-line of the radiation spectrum) of approx. 110 Å to approx. 200 Å, and having approx. 14% to approx. 18% radiation portion in the red wave length range 635 nm to 750 nm and having approx. 7% to approx. 10% radiation portion in the blue wave length range 380 nm to 500 nm, in each case of the radiation power in the wave length range 380 nm to 780 nm for promotion of plant growth is provided.

The invention concerns a high pressure sodium vapour lamp with an arctube made from polychrystalline aluminumoxyd (PCA), a fill of sodium andmercury, as well as xenon as a fill gas.

High pressure sodium vapour lamps of this type are known for their highluminous efficiency in the range of the visible spectrum. This is sobecause the light emission of these lamps takes place in a spectral areawhich corresponds to the maximum of the graph of eye sensitivity. Themost commercially available high pressure sodium vapour-(HPS)-lamps are,therefore, optimized with regard to maximum lamp lumens.

In addition thereto HPS-lamps enjoy a long life and a very small lumendepreciation.

The high luminous efficiency and the long life also render HPS-lampsparticularly useful for application in plant growth promotion, despitethe fact that their spectrum is optimized for the human eye and not forthe favourisation or promotion, respectively, of the plant growthprocess.

From EP 0 364 014) a method is known to optimize the blue part of thespectrum of a HPS-lamp. It is known that this is important to preventplants from growing spindle-meagre and with small leaves. The situationin green houses, however, is such that there is a sufficient amount ofblue light of the sun present already, even if it is cloudy inwintertime.

The European patent application mentioned describes a HPS-lamp which isoptimized with regard to photosynthesis in plants, discloses an optimumsodium-mercury-amalgam-proportion and points out that the PCA arc tubemust be made shorter and wider in order to make this lamp electricallycompatible with present ballasts. It has been discovered, however, thatwhen trying to follow this way the arc tube becomes short to an extentthat increased heat losses at the arc tube ends become apparent andthat, hence, the gain of efficiency with regard to photosynthesis getslost. In addition thereto the low length of the arc tube can result inundesired changes of light distribution in some in the luminairesdestined for plant growth. This implies that the known lamp as describedis usable in connection with specially devised operating systems only.

The object underlying the invention is seen in provision of a highpressure sodium vapour lamp of the type mentioned above which isoptimized with regard to the efficiency of photosynthesis in plants,which is exchangeable with the present lamps, which is compatible withpresent ballasts, starters and luminaries, and which finally whencompared with a conventional HPS-lamp renders a gain as tophotosynthetic effect.

This object is met in accordance with the invention by a high pressuresodium vapour lamp having a PCA-arc tube, a fill consisting of sodium,mercury and xenon as a filling gas, with a sodium weight portion withinthe sodium-mercury-amalgam of approx. 12% to approx. 20% and a xenonfilling pressure in the cold state between approx. 180 Torr and approx.350 Torr, with a D-line reversal width of the radiation spectrum ofapprox. 130 Å to approx. 200 Å, and with approx. 14% to approx. 18%radiation portion in the red wave length area 635 nm to 760 nm and withapprox. 7% to approx. 10% radiation portion in the blue wave length area380 nm to 500 nm, each of a radiation power within the total wave lengtharea 380 nm to 760 nm.

To minimize the heat losses towards the walls of the arc tube it hasbeen recognized as effective to fill the tube with a xenon pressurewhich is as high as possible without endangering the flawless ignitionof the lamps by means of the starters which are certified therefore.This leads to an increase of luminous efficiency of approx. 10%. With alamp in accordance with the invention the xenon pressure should bebrought to a value which is as high as possible but at the same timepermits the lamps to get perfectly ignited by the starters which arecertified for the respective lamp type. In praxi these are superposedpulse igniters usually which show a minimum peak voltage specified independency of lamp power.

In addition thereto the discharge length of the lamp according to theinvention should not deviate by more than 25% from the discharge lengthof conventional HPS-lamps of the same power. If one conforms to this,then optical compatibility with present luminaires is secured. The wallloading (lamp power divided by the wall surface of the arc tube betweenthe electrodes) with conventional HPS-lamps is optimized.Notwithstanding that with higher loading the radiated power increasesthe wall loading must be kept beneath a certain value to secure a longlife. This value usually corresponds to a maximum arc tube temperatureof 1200° and lies between 15 and 25 W/cm² in dependency of nominal lamppower. With the lamp in accordance with the present invention the wallloading may deviate 10% maximally from the value of the correspondingconventional HPS-lamp. The intended increase of photosynthesisefficiency is then realized by optimizing the composition of thesodium-mercury-amalgam and the D-line reversal width (distance of thetops of the two wings of the sodium-D-line of the radiation spectrum) ofapprox. 130 Å to approx. 200 Å. These variables are elected so thatphotosynthesis efficiency of the lamp is optimized. The photosynthesisefficiency or efficacy ,respectively, is defined as

η_(ps)=Φ_(ps) /P _(la)

wherein

Φ_(ps)=K∫V_(ps)(λ)P_(λ)δλ is the photosynthetical effective radiationportion expressed in phytolumen, and

P_(la) is the power distributed within the lamp.

K=1088.4 Phyto-lm·W⁻¹

V_(PS) is the spectrally effective function for the photosynthesis inplants indicated in FIG. 1 and P_(λ) is the lamp spectrum.

Comparison with lamp efficiency is carried out. Lamp efficiency isdefined as

η=Φ/P _(la),

wherein

ΦK_(m)∫V_(x) P_(λ)δλ is the luminous flux expressed in lumen and

K_(m)=683 lm·W⁻¹, and

V_(λ)the degree of spectral sensitivity according to DIN 5031, part 2.

In the drawing is:

FIG. 1 the photosynthetical spectral sensitivity in dependency of wavelength;

FIG. 2 the photosynthetical efficiency of the sodium high pressure lampin dependency of D-line reversal width;

FIG. 3 the luminous efficiency of the sodium-D-high pressure lamp independency of D-line reversal width;

FIG. 4 the gradient of the range which is optimal for photosynthesis andgeneral lighting applications with a sodium high pressure lamp having 12to 20% of weight sodium in the amalgam in dependency of the D-linereversal width.

The dependency of photosynthetical efficiency of the amalgam compositionand the said D-line reversal width in the case of a 400 W-lamp has beentested, as shown in FIG. 2. The data given in FIG. 2 have been obtainedby the manufacture of lamps having different amalgam compositions and bymeasuring their photosynthetically effective radiation portions asfunctions of the D-line reversal width. The data have been recordedduring the starting phase of the lamps by means of a reference ballast.From FIG. 2 it can be easily derived that for all the amalgamcompositions there is a very clear relation between D-line reversalwidth and photosynthesis efficiency.

FIG. 2 shows that the photosynthesis efficiency (phytolumen per Watt) independency of the sodium weight in the amalgam reaches a maximum atvalues between 130 and 200 Å of the D-line reversal width. Thereby withincreasing sodium content the maximum shifts to higher values of theD-line reversal width. For each composition of the sodium amalgam arange for maximum efficiency of photosynthesis can be recognized fromFIG. 2.

In FIG. 3 luminous efficiency (in lumen per Watt) is graphed fordifferent sodium weights as function of the D-line reversal width. Foreach sodium portion a maximum of luminous efficiency can be recognizedthere, which, however, shows a lower dependency of the sodium portion ascompared with photosynthesis efficiency.

The measuring data are composed in table 1 and in FIG. 4 below. Thetable contains the numerical values of the photosynthesis efficiency andof the luminous efficiency in dependency of the sodium weight portion inthe amalgam. In FIG. 4 these values are depicted graphically.

From FIG. 4 it can be clearly recognized that for the same sodiumcontent of 12 to 20% the range of maximal efficiency of photosynthesisas compared with luminous efficiency is characterized by higher D-linereversal widths.

The desired D-line reversal width in praxi can be reached by increasingthe cold spot temperature of the arc tube, whether by varying thedistance of the electrode peak from the face of the tube or byapplication of heat build up bands at the outer surface of both tubeends. With unchanged tube dimensions the operating voltage of the lampis increased thereby. Attention must be paid with regard to the factthat the lamp operating voltage is low enough to ensure a sufficientlife of the lamp. Should the value of the lamp operating voltage be toohigh, a correction onto the desired value can be reached, also withconstant D-line reversal width, by increasing the length of thedischarge arc and reducing the diameter of the tube such that the wallloading of the arc tube per cm² remains constant. It must be observedthat, as mentioned above, the arc length is not changed by more than25%.

TABLE 1 Photosynthetis Luminuous Efficiency Efficiency % Na(Phytolumen/W) (lm/W) 12 110-160 80-100 14 120-170 85-105 16 130-18090-110 18 140-190 90-110 20 150-200 95-115

An example has been carried out for the case of a 400 W-amp.

Thereby the arc tube had the following dimensions:

inner tube length: 107 mm

inner diameter of the arc tube: 8.1 mm

Arc length: 84.6 mm

Wall thickness: 0.75 mm

The amalgam composition showed 16 percent of weight sodium. The coldxenon pressure within the arc tube was 308 Torr. The cold spottemperature was set to 120 V by adjustment of the distance betweenelectrode point and the face of the arc tube to a value of 17.0 mm.

In the following table 2 the measured characteristics of this lamp arecompared with those of a conventional HPS-lamp with increased fillingpressure. By the term “conventional” is meant a commercially availablesodium high pressure lamp for general lighting applications. Such a lampcorresponds to SYLVANIA Type SHP-TS 400W.

TABLE 2 Comparison lamp example according to the invention- inventionalHPS-lamp with increased filling pressure Conventional high pressuresodium Lamp example lamp D-line reversal 130 109 width (Å) Operatingvoltage 120 98 (V) Lamp power(W) 424 392 Luminous 58028 53473 efficiency(lm) Phytolumen (lm) 128729 115648 PAR (μmol/s) 715 644 Pblue (W) 11,49,6 Pred (W) 17,3 14,5

The lamp according to the invention described above as an example wastested in praxi. In doing so four varieties of cucumber in a room sealedagainst daylight have been irradiated and brought up with lampscorresponding to the example given above. The duration of irradiationhas been sixteen hours per day and the duration of growth one month. Forcomparison the same cucumber varieties have been brought up under thesame conditions by an irradiation stemming from conventional HPS-lampsas mentioned above already, namely of the type SYLVANIA SHP-TS 400W. Thegreater lumen output of the lamps according to the invention wascompensated by enlargement of the distance between the lamps and theplants in order to obtain the same photosynthetically effectiveradiation intensity at the level of plant growth or at the substrate,respectively, as in the case of the conventional lamps.

In the following table 3 the test results are listed which have beenobtained by the said bringing up of the cucumber plants. From table 3 itwill be apparent that the irradiation achieved with the lamps inaccordance with the invention significantly promotes the growth of thecucumber plants as can be derived from the obtained plant dimensions,plant weights and leaf sizes detected.

TABLE 3 Lamp according to invention Conventional HPS-lamp CucumberLength Number of Leaf size weight length Number of Leaf size weightvariety cm leaves cm g cm leaves cm g Sabrina 128 11.6 21.6/28.7 140.3116.4 10.6 20.1/26   112.6 Dugan 133.7 12.1 21.5/27.2 143 113.2 10.819.3/24.1 105.4 Korinda 121 11.9 21.8/28.2 139.9 106.7 10.7 19.5/24.4104.1 Bellissima 123.9 11.4 21.3/27   139.4 104.7 10 18.7/23.1 101.1

Of each of the four cucumber varieties mentioned eighteen specimensrespectively have been irradiated by the lamps according to theinvention and twenty-two specimens by the conventional lamps.

It will be understood that the indications of the number of leaves andof the weights which deviate from full numbers which, hence, representdecimal fractions, have been produced by forming averages. The averageweights have been obtained without roots at the end of the month ofgrowth each. At this early point in time a formation of cucumbers cannotbe expected.

What is claimed is:
 1. High pressure sodium vapour lamp having a fillconsisting of sodium, mercury and xenon in an arc tube with a sodiumweight portion within the sodium-mercury-amalgam of approx. 12% toapprox. 20%, having a xenon filling pressure in the cold state betweenapprox. 180 Torr and approx. 350 Torr, having a D-line reversal width(distance of the tops of both wings of the sodium-D-line of theradiation spectrum) of between approx. 100 Å and approx. 200 Å, andhaving between approx. 14% and approx. 18% radiation portion in the redwave lengths range 635 nm to 750 nm and having between approx. 7% andapprox. 10% radiation portion in the blue wave lengths range 380 nm to500 nm, in each case of the radiation power in the wave length range 380nm to 780 nm for promotion of plant growth.
 2. High pressure sodiumvapour lamp according to claim 1, characterized in that the sodiumweight portion in the amalgam is between 14% and 18% and the D-linereversal width is between 120 Å and 190 Å.
 3. High pressure sodiumvapour lamp according to claim 2, characterized in that the sodiumweight portion in the amalgam is 16% and the D-line reversal width isbetween 130 Å and 180 Å.